Reading circuit for a magnetic field sensor with sensitivity calibration, and related reading method

ABSTRACT

A reading circuit for a magnetic-field sensor, generating an electrical detection quantity as a function of a detected magnetic field and of a detection sensitivity, is provided with an amplification stage, which is coupled to the magnetic-field sensor and generates an output signal as a function of the electrical detection quantity and of an amplification gain. In particular, the amplification gain is electronically selectable, and the reading circuit is moreover provided with a calibration stage, integrated with the amplification stage and configured so as to vary a value of the amplification gain in such a way as to compensate a variation of the detection sensitivity with respect to a nominal sensitivity value.

BACKGROUND

1. Technical Field

The present relates to a reading circuit for a magnetic-field sensor,for example an anisotropic magnetoresistive (AMR) magnetic sensor, withcalibration of the sensitivity of the sensor, and to a correspondingcalibration method.

2. Description of the Related Art

Magnetic-field sensors, in particular AMR magnetic sensors, are used ina plurality of applications and systems, for example in compasses, insystems for detecting ferrous materials, in the detection of currents,and in a wide range of other applications, thanks to their capacity ofdetecting natural magnetic fields (for example, the Earth's magneticfield) and magnetic fields generated by electrical components (such aselectrical or electronic devices and lines traversed by electriccurrent).

As it is known, the phenomenon of anisotropic magnetoresistivity occurswithin particular ferrous materials, which, when subjected to anexternal magnetic field, undergo a variation of resistivity as afunction of the characteristics of the external magnetic field. Usually,these materials are applied in the form of thin strips so as to formresistive elements, and the resistive elements thus formed areelectrically connected to form a bridge structure (typically aWheatstone bridge).

It is moreover known to manufacture AMR magnetic sensors with standardsemiconductor micromachining techniques, as described, for example, inU.S. Pat. No. 4,847,584. In particular, each magnetoresistive elementcan be formed by a film of magnetoresistive material, such as forexample permalloy (i.e., a ferromagnetic alloy containing iron andnickel), deposited to form a thin strip on a substrate made ofsemiconductor material, for example silicon.

When an electric current is made to flow through a magnetoresistiveelement, the angle θ between the direction of magnetization of the samemagnetoresistive element and the direction of the current flow affectthe effective value of resistivity of the magnetoresistive element sothat, as the value of the angle θ varies, the value of electricalresistance varies (in detail, the variation follows a law of the cos²θtype). For example, a direction of magnetization parallel to thedirection of the current flow results in a maximum resistance value tothe passage of current through the magnetoresistive element, whereas adirection of magnetization orthogonal to the direction of the currentflow results in a minimum resistance value to the passage of currentthrough the magnetoresistive element.

Usually, AMR magnetic sensors moreover include coils, integrated in thesensors, the so-called “offset straps”, which are designed to generate,when traversed by a current of an appropriate value, a magnetic fieldthat couples in the direction of detection of the sensors; in thisregard, see for example U.S. Pat. No. 5,247,278. These offset straps arenormally used for operations of compensation of the offsets present inthe sensors (on account of mismatches in the values of the correspondingelectrical components) and self-test operations. In particular, thevalue of the electrical quantities at output from the sensors are inthis case a function both of the external magnetic field to be detectedand of the magnetic field generated internally as a result of thecurrent circulating in the offset straps (which is indeed detected bythe magnetoresistive elements). The offset straps are constituted byturns of conductive material, for example metal, which are arranged onthe same substrate on which the magnetoresistive elements of the sensorare provided, being electrically insulated from, and set in theproximity of, the same magnetoresistive elements.

In particular, the Wheatstone-bridge detection structure of an AMRmagnetic sensor includes magnetoresistive elements having ideally thesame resistance value, and such as to form diagonal pairs of equalelements, which react in an opposite way with respect to one another tothe external magnetic fields, as shown schematically in FIG. 1 (where Iis the electric current flowing in the magnetoresistive elements and Rthe common resistance value).

If a supply voltage V_(s) is applied at the input of the bridgedetection structure (in particular to first two terminals of the bridge,operating as input terminals), in the presence of an external magneticfield H_(e), a variation of resistance ΔR of the magnetoresistiveelements and a corresponding variation of the voltage drop value on thesame magnetoresistive elements occur. In fact, the external magneticfield H_(e) determines a variation of the direction of magnetization ofthe magnetoresistive elements. This results in an unbalancing of thebridge, which takes the form of a voltage variation ΔV at output fromthe bridge circuit (in particular between the remaining two terminals ofthe bridge, operating as output terminals). Since the direction of theinitial magnetization of the magnetoresistive elements is knownbeforehand, as a function of the voltage variation ΔV it is thuspossible to determine the component of the external magnetic fieldacting in the direction of sensitivity of the magnetic sensor (it isthus possible, using three magnetic sensors with directions ofsensitivity orthogonal to one another, to determine the modulus anddirection of the external magnetic field).

In particular, in order to detect unbalancing of the Wheatstone bridgeand generate an output signal indicating the characteristics of theexternal magnetic field to be measured, a reading circuit (or front-end)is normally used, which is coupled to the output of the AMR magneticsensor, and includes, for example, an instrumentation amplifier. The AMRmagnetic sensor and the associated reading circuit together form amagnetic-field sensor device, which supplies at output an electricalsignal as a function of the detected magnetic field, and has a giveninput/output response, due in part to the sensitivity of the bridgedetection structure, and in part to the gain of the associated readingfront-end.

In a known way, the sensitivity of AMR magnetic sensors, i.e., themagnitude of the electrical response supplied by the correspondingbridge detection structure as a function of the external magnetic fieldto be detected, normally has a high variability (or spread), which caneven reach 40% with respect to the nominal value. This spread is due,for example, to the intrinsic process variations associated to themanufacturing of the sensors.

Consequently, a same external magnetic field can generate electricalsignals the value of which can vary considerably even between sensorsbelonging to one and the same production lot. Such a spread in thesensitivity of AMR magnetic sensors is not desirable, in particular inthose applications that require an accurate measurement of the magneticfield to be detected, such as, for example, in magnetometers.

Therefore, techniques for calibration of AMR magnetic sensors have beenproposed, designed to reduce or at least limit the spread of thesensitivity of the corresponding detection structures.

For example, a calibration technique envisages the use of processes of“laser trimming” during the manufacturing process of the AMR magneticsensors, i.e., the use of techniques of laser removal for adjusting thevalues of the electronic components that constitute the sensors. Inparticular, within an external environment with controlled magneticfield, the electrical characteristics of the sensor are physicallyadjusted in such a way that it will supply at output an electricalquantity having a value corresponding to that of the external magneticfield, irrespective of the process variations that might have alteredthe sensitivity thereof.

This technique, however, in addition to being complex and costly toimplement (in so far as it requires costly testing and calibrationequipment), requires an accurate control of the magnetic field presentin the area surrounding the sensors during the calibration operations.This accurate control may, however, prove difficult to achieve onaccount, for example, of parasitic magnetic fields generated by thetesting machinery or coming from the manufacturing environment.

The techniques of calibration of AMR magnetic sensors that have so farbeen proposed are hence not altogether satisfactory, and frequently areunable to ensure the desired results.

BRIEF SUMMARY

An embodiment provides a technique for calibrating the sensitivity of anAMR magnetic sensor, being free from the disadvantages of the known art,highlighted previously.

In an embodiment, a reading circuit for a magnetic-field sensor, saidmagnetic-field sensor being designed to generate an electrical detectionquantity as a function of a detected magnetic field and of a detectionsensitivity, comprises an amplification stage, coupled to saidmagnetic-field sensor and configured to generate an output signal as afunction of said electrical detection quantity and of an amplificationgain, wherein said amplification gain is electronically selectable, andby comprising a calibration stage, integrated with said amplificationstage and configured to vary a value of said amplification gain so as tocompensate for a variation of said detection sensitivity with respect toa nominal sensitivity value. In an embodiment, said calibration stage isconfigured to: detect at least one value associated to said outputsignal upon detection, by said magnetic-field sensor, of a controlledmagnetic field, of a known value; determining an effective value of saiddetection sensitivity, as the result of said variation of sensitivity,as a function of said value associated to said output signal; andvarying the value of said amplification gain based on said effectivevalue of said detection sensitivity. In an embodiment, saidmagnetic-field sensor is provided with at least one firstmagnetoresistive element and with a magnetization element operativelycoupled to said at least one first magnetoresistive element; and whereinsaid calibration stage is configured so as to cause generation of saidcontrolled magnetic field as the result of an excitation current sentthrough said magnetization element. In an embodiment, said calibrationstage is configured to: acquire at least a first value of said outputsignal, in the presence of an external magnetic field and in the absenceof said controlled magnetic field; acquire at least a second value ofsaid output signal, in the presence both of said external magnetic fieldand of said controlled magnetic field; and jointly process said firstand second values of said output signal in order to determine saideffective value of said detection sensitivity. In an embodiment, saidcalibration stage is configured to determine a difference between saidfirst value and said second value of said output signal in order todetermine said effective value of said detection sensitivity as afunction of the value of said controlled magnetic field, irrespective ofthe value of said external magnetic field. In an embodiment, saidamplification stage comprises an amplifier unit, having at least oneinput designed to receive said electrical detection quantity and atleast one output designed to supply said output signal; and wherein saidcalibration stage comprises a gain-variation unit, coupled to saidamplifier unit and configured to vary a gain thereof between the inputand the output. In an embodiment, said amplifier unit comprises a gainnetwork, coupled to said at least one input and to said at least oneoutput, and said gain-variation unit comprises an adjustable-impedanceunit, which is coupled to said gain network and has a selectableimpedance value; and wherein said calibration stage is configured toprovide a gain-control signal to said gain-variation unit for selectingthe value of said impedance. In an embodiment, said magnetic-fieldsensor is an AMR magnetic sensor provided with further magnetoresistiveelements, arranged with said at least one first magnetoresistive elementto form a bridge detection structure; wherein said electrical detectionquantity is a unbalancing signal of said bridge detection structure. Inan embodiment, an electronic device comprises a magnetic-field sensor,and a reading circuit, coupled to said magnetic-field sensor, saidelectronic device further comprising a control unit, coupled to saidreading circuit for receiving said output signal. In an embodiment, saidreading circuit is made as an ASIC (application-specific integratedcircuit), and is housed in a same package together with a dieintegrating said magnetic-field sensor.

In an embodiment, a method for reading a magnetic-field sensor, saidmagnetic-field sensor being designed to generate an electrical detectionquantity as a function of a detected magnetic field and of a detectionsensitivity, comprises the step of generating, by means of a readingcircuit, coupled to said magnetic-field sensor and having anamplification gain, an output signal as a function of said electricaldetection quantity and of said amplification gain, and varying a valueof said amplification gain so as to compensate for a variation of saiddetection sensitivity with respect to a nominal sensitivity value. In anembodiment, said varying comprises: detecting at least one valueassociated to said output signal upon detection, by said magnetic-fieldsensor, of a controlled magnetic field, of a known value; determining aneffective value of said detection sensitivity, as the result of saidvariation of sensitivity, as a function of said value associated to saidoutput signal; varying the value of said amplification gain based onsaid effective value of said detection sensitivity. In an embodiment,said magnetic-field sensor is provided with at least one firstmagnetoresistive element and with a magnetization element operativelycoupled to said at least one first magnetoresistive element; and whereinsaid step of detecting at least one value comprises: acquiring at leasta first value of said output signal, in the presence of an externalmagnetic field and in the absence of said controlled magnetic field;acquiring at least a second value of said output signal, in the presenceboth of said external magnetic field and of said controlled magneticfield; and jointly processing said first value and said second value ofsaid output signal in order to determine said effective value of saiddetection sensitivity. In an embodiment, jointly processing comprises:computing a difference between said first value and said second value ofsaid output signal in order to determine said effective value of saidsensitivity, as a function of the value of said controlled magneticfield irrespective of the value of said external magnetic field. In anembodiment, said reading circuit is made as a ASIC (Application SpecificIntegrated Circuit), and is housed in a same package together with a dieintegrating said magnetic-field sensor.

In an embodiment, a reading circuit comprises: an amplification stageconfigured to receive a magnetic field detection signal of a magneticfield sensor and to generate an output signal as a function of themagnetic field detection signal and of an amplification gain of theamplification stage; and a calibration stage configured to control theamplification gain of the amplification stage based on an indication ofa detection sensitivity of the magnetic field sensor. In an embodiment,said calibration stage is configured to: measure a value of the outputsignal associated with a controlled magnetic field having a known value;determine an effective value of said detection sensitivity as a functionof said measured value; and control the amplification gain based on saideffective value of said detection sensitivity. In an embodiment, saidcalibration stage is configured to generate an excitation current toexcite a magnetization element of the magnetic field sensor and theamplification stage is configured to electrically couple to a firstmagnetoresistive element of the magnetic field sensor. In an embodiment,said calibration stage is configured to: measure at least a first valueof said output signal, in the presence of an external magnetic field andin the absence of a controlled magnetic field; measure at least a secondvalue of said output signal, in the presence both of said externalmagnetic field and of said controlled magnetic field; determine aneffective value of said detection sensitivity as a function of saidfirst and second measured values; and control the amplification gainbased on said effective value of said detection sensitivity. In anembodiment, said calibration stage is configured to determine adifference between said first measured value and said second measuredvalue of said output signal. In an embodiment, said amplification stagecomprises an amplifier, having at least one input configured to receivesaid magnetic field detection signal and at least one output configuredto supply said output signal; and wherein said calibration stagecomprises a gain-variation unit, coupled to said amplifier andconfigured to vary a gain of the amplifier between the at least oneinput and the at least one output of the amplifier. In an embodiment,said amplifier comprises a gain network, coupled to said at least oneinput and to said at least one output, and said gain-variation unitcomprises an adjustable-impedance unit coupled to said gain network andhaving a selectable impedance value, wherein said calibration stage isconfigured to provide a gain-control signal to said gain-variation unitfor selecting a value of said selectable impedance. In an embodiment,the magnetic field sensor is an anisotropic magnetoresistive (AMR)magnetic sensor, the amplification stage is configured to electricallycouple to a bridge detection structure formed from a plurality ofmagnetoresistive elements of the magnetic field sensor, and saidmagnetic detection signal is an unbalancing signal of said bridgedetection structure.

In an embodiment, a system comprises: a magnetic-field sensor configuredto generate a magnetic field detection signal as a function of one ormore magnetic fields; and a reading circuit having: an amplificationstage coupled to the magnetic-field sensor and configured to generate anoutput signal as a function of the magnetic field detection signal andof an amplification gain of the amplification stage; and a calibrationstage configured to control the amplification gain of the amplificationstage based on an indication of a detection sensitivity of the magneticfield sensor. In an embodiment, the system further comprises a controlunit, coupled to said reading circuit and configured to receive saidoutput signal. In an embodiment, said reading circuit is anapplication-specific integrated circuit housed in a same packagetogether with a die comprising said magnetic-field sensor. In anembodiment, said calibration stage is configured to: measure a value ofthe output signal associated with a controlled magnetic field having aknown value; determine an effective value of said detection sensitivityas a function of said measured value; and control the amplification gainbased on said effective value of said detection sensitivity. In anembodiment, said calibration stage is configured to generate anexcitation current to excite a magnetization element of the magneticfield sensor and the amplification stage is configured to electricallycouple to a first magnetoresistive element of the magnetic field sensor.In an embodiment, wherein said calibration stage is configured to:measure at least a first value of said output signal, in the presence ofan external magnetic field and in the absence of a controlled magneticfield; measure at least a second value of said output signal, in thepresence both of said external magnetic field and of said controlledmagnetic field; determine an effective value of said detectionsensitivity as a function of said first and second measured values; andcontrol the amplification gain based on said effective value of saiddetection sensitivity. In an embodiment, said calibration stage isconfigured to determine a difference between said first measured valueand said second measured value of said output signal.

In an embodiment, a method comprises: receiving a signal of amagnetic-field sensor, the received signal being a function of adetected magnetic field and of a detection sensitivity of themagnetic-field sensor; controlling an amplification gain of a readingcircuit to compensate for a variation of said detection sensitivity froma nominal detection sensitivity value; and generating an output of thereading circuit as a function of said received signal and of saidamplification gain. In an embodiment, said controlling the amplificationgain comprises: measuring a value of the output signal associated with acontrolled magnetic field having a known value; determining an effectivevalue of said detection sensitivity as a function of said measuredvalue; and controlling the amplification gain based on said effectivevalue of said detection sensitivity. In an embodiment, said measuring avalue of the output signal associated with a controlled magnetic fieldcomprises generating an excitation current to excite a magnetizationelement of the magnetic field sensor. In an embodiment, said controllingthe amplification gain comprises: measuring at least a first value ofsaid output signal, in the presence of an external magnetic field and inthe absence of a controlled magnetic field; measuring at least a secondvalue of said output signal, in the presence both of said externalmagnetic field and of said controlled magnetic field; determining aneffective value of said detection sensitivity as a function of saidfirst and second measured values; and controlling the amplification gainbased on said effective value of said detection sensitivity. In anembodiment, said determining said effective value comprises determininga difference between said first measured value and said second measuredvalue of said output signal.

In an embodiment, a system comprises: means for generating a magneticfield detection signal as a function of one or more magnetic fields;means for generating an output signal as a function of the magneticfield detection signal and an amplification gain; and means forcontrolling the amplification gain based on an indication of a detectionsensitivity of the means for generating the magnetic field detectionsignal. In an embodiment, the system comprises an application-specificintegrated circuit including the means for generating the output signaland the means for controlling the amplication gain in a same packagetogether with a die comprising said means for generating the magneticfield detection signal. In an embodiment, said means for controlling theamplification gain is configured to: measure at least a first value ofsaid output signal, in the presence of an external magnetic field and inthe absence of a controlled magnetic field; measure at least a secondvalue of said output signal, in the presence both of said externalmagnetic field and of said controlled magnetic field; determine aneffective value of said detection sensitivity as a function of saidfirst and second measured values; and control the amplification gainbased on said effective value of said detection sensitivity. In anembodiment, said means for controlling the amplification gain isconfigured to determine a difference between said first measured valueand said second measured value of said output signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of this disclosure, embodiments thereof arenow described, purely by way of non-limiting example, with reference tothe attached drawings, wherein:

FIG. 1 shows a simplified electrical diagram of an AMR magnetic sensorof a known type, with a Wheatstone-bridge detection structure;

FIG. 2 shows a simplified diagram of a reading circuit for amagnetic-field sensor, in particular an AMR magnetic sensor, including acalibration stage according to one embodiment;

FIG. 3 is a flowchart corresponding to operations envisaged by acalibration method implemented in the circuit of FIG. 2; and

FIG. 4 shows a simplified block diagram of an electronic deviceincluding the reading circuit and the magnetic-field sensor of FIG. 2.

DETAILED DESCRIPTION

In the following description, certain details are set forth in order toprovide a thorough understanding of various embodiments of devices,methods and articles. However, one of skill in the art will understandthat other embodiments may be practiced without these details. In otherinstances, well-known structures and methods associated with, forexample, magnetic sensors, amplifiers, etc., have not been shown ordescribed in detail in some figures to avoid unnecessarily obscuringdescriptions of the embodiments.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, such as“comprising,” and “comprises,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout this specification to “one embodiment,” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment,” or“in an embodiment” in various places throughout this specification arenot necessarily referring to the same embodiment, or to all embodiments.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments to obtainfurther embodiments.

The headings are provided for convenience only, and do not interpret thescope or meaning of this disclosure or the claims.

The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale. For example, the shapes of various elementsand angles are not drawn to scale, and some of these elements may beenlarged and positioned to improve drawing legibility. Further, theparticular shapes of the elements as drawn are not necessarily intendedto convey any information regarding the actual shape of particularelements, and have been selected solely for ease of recognition in thedrawings. Geometric references are not intended to refer to idealembodiments. For example, a reference to square-shaped does not meanthat an element has a geometrically perfect square shape.

An embodiment applies to a magnetic-field sensor, for example, an AMRmagnetic sensor of the type described with reference to FIG. 1, andenvisages implementation of a reading circuit, associated to themagnetic-field sensor, which comprises: a variable and selectable gainstage; and a calibration stage, configured for varying the gain valueimplemented by the gain stage in such a way as to compensate, and inparticular reduce, spread of sensitivity of the magnetic-field sensordue, for example, to the manufacturing process of the samemagnetic-field sensor. In other words, the reading circuit associated tothe magnetic-field sensor is provided with a function of calibration ofthe sensitivity of the sensor in such a way that the user will perceivean overall response of the sensor device (understood as the assembly ofthe sensor and of the corresponding reading circuit) that corresponds toa nominal or expected response (i.e., a data sheet response).

An embodiment envisages use, for the aforesaid operation of calibration,of an offset coil (the so-called “offset strap”) integrated in thestructure of the AMR magnetic sensor (see the foregoing description) inorder to generate an internal magnetic field of a known and controlledvalue, in the proximity of the magnetoresistive elements of themagnetic-field sensor. The response of the sensor device to thismagnetic field of a controlled value can in fact be used for determiningthe spread of the effective sensitivity of the magnetic-field sensor andvary the gain of the associated reading circuit accordingly.

In detail, FIG. 2 is a schematic illustration of a sensor deviceaccording to one embodiment, designated as a whole by 1, including amagnetic-field sensor (in particular an AMR magnetic sensor), designatedby 2 and represented schematically by means of its Wheatstone-bridgeequivalent circuit, and a reading circuit (or front-end) 3, associatedto the magnetic-field sensor 2.

In particular, the magnetic-field sensor 2 comprises a Wheatstone-bridgedetection structure, with four magnetoresistive elements 2 a-2 d, forexample, constituted by strips of a thin film of magnetoresistivematerial, such as permalloy, having a same resistance value at rest R(in the absence of external magnetic fields) and designed to undergo inpairs one and the same variation ΔR in the presence of an externalmagnetic field H_(e) (the pairs are formed by the diagonally facingelements in the bridge; namely, a first pair is formed by themagnetoresistive elements 2 a and 2 c, and a second pair by themagnetoresistive elements 2 b and 2 d).

The Wheatstone-bridge detection structure has a first input terminalIn₁, connected to the positive pole of a supply source, which supplies asupply voltage V_(s), and a second input terminal In₂, connected to thenegative pole of the same supply source (for example, coinciding with aground terminal of the reading circuit 3). The Wheatstone-bridgedetection structure moreover has a first output terminal Out₁ and asecond output terminal Out₂, present across which is the electricalunbalancing signal (i.e., the voltage variation ΔV), which is afunction, in particular, of the characteristics of the external magneticfield H_(e).

The magnetic-field sensor 2 further comprises an offset coil 4(represented schematically in FIG. 2 as a resistor, and commonly knownas “offset strap”), arranged so as to be magnetically coupled to themagnetoresistive elements 2 a-2 d and electrically connected to acurrent generator 5, designed to supply an excitation current I_(exc),of a controlled value, to the same offset strap 4. In a known way, andas has been discussed above, the offset strap 4 is integrated in themagnetic-field sensor 2 (for example, being provided on the same siliconsubstrate as that on which the magnetoresistive elements 2 a-2 d areprovided) and is designed to generate, as a function of the value of theexcitation current I_(exc), an internal magnetic field H_(i), of a knownvalue.

The reading circuit 3 comprises an amplification stage 6 (shownschematically in FIG. 2), which is electrically coupled to the output ofthe magnetic-field sensor 2 and includes, for example, an operationalamplifier, designated by 8, of a fully differential type (i.e., havingdifferential inputs and outputs). The amplifier 8 has a non-invertinginput 8 a connected to the first output terminal Out₁ of themagnetic-field sensor 2, and an inverting input 8 b connected to thesecond output terminal Out₂. The amplifier 8 moreover has an invertingoutput 8 c and a non-inverting output 8 d, present across which is anoutput signal Vout, which is a function of the voltage variation ΔVsupplied by the magnetic-field sensor 2.

The amplification stage 6 comprises a first circuit branch and a secondcircuit branch, which are substantially the same, are connectedrespectively to the non-inverting and inverting inputs 8 a, 8 b of theamplifier 8, and each of which provides a gain network, in this case ofthe capacitive type (evidently, in a different embodiment, a resistivegain network may be provided).

As illustrated, each branch comprises a first gain capacitor 9, having afirst terminal connected to a respective output terminal Out₁, Out₂(according to the circuit branch considered) of the magnetic-fieldsensor 2, and a second terminal connected to a respective non-inverting8 a or inverting 8 b input (once again, according to the circuit branchconsidered) of the amplifier 8. The first gain capacitor 9 has acapacitance C₁. Each circuit branch further comprises a second gaincapacitor 10, having a first terminal connected to a respectivenon-inverting 8 a or inverting 8 b input (according to the circuitbranch considered) of the amplifier 8, and a second terminal connectedto a respective inverting 8 c or non-inverting 8 d output (once again,according to the circuit branch considered) of the amplifier 8. Thesecond gain capacitor 10 has a capacitance C₂.

The reading circuit 3 further comprises a calibration stage 12,configured so as to implement a calibration algorithm (described indetail hereinafter) for compensating variations of sensitivity of thedetection structure of the magnetic-field sensor 2, and in particularfor reducing the effects of possible spread, due, for example, to themanufacturing process.

In detail, the calibration stage 12 comprises a firsttrimmable-capacitor (or “captrim”) unit 14 a and a secondtrimmable-capacitor (or “captrim”) unit 14 b, which are connected inparallel to the first gain capacitor 9 respectively of the first andsecond circuit branches and act as elements for variation of the gain ofthe amplification stage 6. Each captrim unit 14 a, 14 b has: a firstterminal and a second terminal, between which an overall capacitance ofvariable value C_(x) is present; and a control terminal, on which itreceives a gain-control signal S_(g), which determines the value of theaforesaid overall capacitance; the gain-control signal S_(g) may be an-bit digital signal (b₀, b₁, b₂, . . . b_(n)), where n is, for example,equal to six. Clearly, in a different embodiment, where a resistive gainnetwork is used, use of a resistor with variable and selectableresistance could be envisaged.

In greater detail, each captrim unit 14 a, 14 b is made by a respectivebank of elementary capacitors 14 a ₀-14 a _(n), 14 b ₀-14 b _(n), whichare selectively connected in parallel to one another, and each of whichis connected in series to a respective enabling switch 15, controlled bya respective bit b₀-b_(n) of the gain-control signal S_(g). In this way,as a function of the bit configuration of the gain-control signal S_(g),the configuration of the elementary capacitors 14 a ₀-14 a _(n), 14 b₀-14 b _(n) connected in parallel varies, and hence the value of theoverall capacitance C_(x) of the captrim units 14 a, 14 b also varies.

In particular, it is evident that the gain G of the amplification stage6, which determines the value of the output signal Vout, is given by

G=−(C ₁ +C _(x))/C ₂

where the overall capacitance C_(x) is given by the sum of thecapacitances of the elementary capacitors 14 a ₀-14 a ₀, 14 b ₀-14 b_(n) that are each time connected in parallel in the respective bank ofcapacitors. The gain G is hence variable, and electronically selectable(so as to be incremented or decremented), as a function of thegain-control signal S_(g).

The calibration stage 12 further comprises: an analog-to-digitalconverter (ADC) 18, which has inputs connected to the outputs 8 c, 8 dof the amplifier 8 and is designed to convert from analog to digital theoutput signal Vout supplied by the amplifier 8 (the ADC 18 operates withm bits, where m is, for example, equal to twelve); and a processing unit19 (for example, including a microprocessor or a microcontroller, or asimilar computing tool), connected to the output of the ADC 18 anddesigned to implement, by means of an appropriate control logic, thesteps of the calibration algorithm that will be described hereinafter.

In particular, the processing unit 19, as a function of the value of theoutput signal Vout, determines the value of the gain-control signalS_(g) to be sent to the first captrim unit 14 a and to the secondcaptrim unit 14 b, for varying the gain G of the amplification stage 6.

The processing unit 19 also generates a current-control signal S_(i),which it sends to the current generator 5, for controlling supply of theexcitation current I_(exc) to the offset strap 4 integrated in themagnetic-field sensor 2, and in particular for controlling generation,by means of the same offset strap 4, of the internal magnetic fieldH_(i) of known and controlled value. In a different embodiment, thecurrent generator 5 can be controlled by a control unit external to thereading circuit 3 (for example, a control unit of the electronicapparatus that incorporates the sensor device 1), and the processingunit 19 receives from this external control unit the information on thevalue of the internal magnetic field H_(i) generated, when required, bymeans of the offset strap 4.

As illustrated in FIG. 3, an embodiment of an algorithm for calibrationof the magnetic-field sensor 2 envisages the operations described inwhat follows, which may be carried out within the calibration stage 12,for example by the corresponding processing unit 19.

In an initial step 20, the operations of calibration and compensation ofthe variations of sensitivity of the magnetic-field sensor 2 begin.Start of these operations may for example occur: at the end of themanufacturing process for the sensor and the corresponding readingelectronics; following upon a command imparted by a user; following upona command received from an external electronic unit; or else followingupon turning-on of the electronic apparatus incorporating the sensordevice 1 (according to the specific application and/or the userrequirements).

Next at step 21, the processing unit 19 issues a command for executionof a first measurement of the output signal Vout supplied by theamplifier 8 coupled to the magnetic-field sensor 2, in the presence ofjust the external magnetic field H_(e) (which is due, for example, tothe Earth's magnetic field, and the value of which is not knownbeforehand), i.e., without there being applied any excitation to theoffset strap 4 (the internal magnetic field H_(i) is hence zero).

In this way, a first sample Vout₁ of the output signal Vout is acquired,the value of which is given by the following expression:

Vout₁ =H _(e) ·S·G

where S is the value of sensitivity of the magnetic-field sensor 2,which may possibly differ from a nominal value on account of processspread, and G is the gain of the amplification stage 6, given by theexpression given above; namely,

G=−(C ₁ +C _(x))/C ₂

Next at step 22, the internal magnetic field H_(i) of known andcontrolled value is generated in the magnetic-field sensor 2 bysupplying the excitation current I_(exe) to the offset strap 4integrated in the sensor.

A second measurement of the output signal Vout is then carried out atstep 23, and a second sample Vout₂ of the same output signal Vout isacquired. Given that the magnetoresistive elements 2 a-2 d of themagnetic-field sensor 2 sense in this case both the external magneticfield H_(e) and the internal magnetic field H_(i), the second sampleVout₂ is given by the following expression:

Vout₂=(H _(e) +H _(i))·S·G

The processing unit 19 then computes at step 24 a difference Diffbetween the first sample Vout₁ and the second sample Vout₂ acquiredpreviously (this difference may be conveniently carried out digitally):

Diff=Vout₂ −Vout₁=(H _(e) +H _(i))·S·G−H_(e) ·S·G=H _(i) ·S·G

The difference Diff is hence a function of just the internal magneticfield H_(i) and is independent of the value of the external magneticfield H_(e). Since the value of the internal magnetic field H_(i) isknown, it is possible to act on the value of gain G of the amplificationstage 6 for compensating possible variations of the sensitivity S of themagnetic-field sensor 2 to obtain an expected value of the differenceDiff (i.e., a value that would be obtained with the nominal value ofsensitivity S).

In particular, the product S·G defines an overall response of the sensordevice 1 (perceived by the user) between the input, in this caseconstituted by the value of the internal magnetic field H_(i), and theoutput, in this case constituted by the value of the difference Diff.

In an embodiment, the method is employed to cause this overall responseto have a nominal or expected value, as per design specifications (or atleast to depart as little as possible from this nominal value, within acertain tolerance as per specifications). To achieve this purpose, anembodiment acts on the value of gain G of the amplification stage 6 inorder to compensate the presence of spread on the sensitivity value S ofthe magnetic-field sensor 2.

Consequently, in a step 25 the method envisages verifying whether thevalue of the difference Diff corresponds, or not, to the expected value,i.e., whether correct compensation of the variation of the value ofsensitivity S with respect to the nominal value has been achieved.Alternatively, this check may verify that the difference Diff does notdepart from the expected value by more than a threshold value (the valueof which may depend upon the type of application and the tolerable levelof spread on the sensitivity value S).

If it is found that the difference Diff has the expected value (or doesnot depart from the expected value by more than the threshold), thealgorithm goes on to step 26 to the end of the calibration operations.For example, a message can then be issued to the user indicating thatthe magnetic-field sensor 2 is calibrated, and that, in a subsequentmeasurement operation (where the generation of the internal magneticfield H is again disabled), the value of the output signal Vout willeffectively indicate the external magnetic field H_(e) to be measured.

Instead, the finding that at step 25 the difference Diff does not havethe expected value entails modification of the value of the gain G ofthe amplification stage 6. Consequently, in a step 27 following uponstep 25, the processing unit 19 implements a variation of the value ofgain G, in particular by varying the gain-control signal S_(g) sent tothe captrim units 14 a, 14 b (see the foregoing description). Ingeneral, the variation of gain G may occur in discreteincrement/decrement steps so as to obtain calibration by successiveapproximations, or else, the new value of gain G may be determined (bymeans of a function or a tabular relation) as a function of thedeviation of the value of the difference Diff from the expected value.

In particular, the effective value of the sensitivity of themagnetic-field sensor 2, designated by S, can be determined as afunction of the current value of gain G implemented in the readingcircuit 3, the value of the internal magnetic field H_(i), and the valueof the difference Diff

$\overset{\_}{S} = \frac{Diff}{H_{i} \cdot G}$

The new value of gain G′ may thus be determined as a function of theaforesaid effective value of the sensitivity S and of the nominal valueof the same sensitivity S (and, for example, of the tolerance that it isdesired to obtain with respect to the same nominal value)

G′=f(S, s )

Once the new value of gain G′ has been determined and the same gainvalue has been implemented within the amplification stage 6 (by means ofan appropriate reconfiguration of the captrim units 14 a, 14 b), thealgorithm hence returns to the initial step (step 21), for execution ofa new session of measurement and determination of a new value for thedifference Diff, with the new value of gain G′ selected.

The algorithm is thus repeated cyclically until correct compensation ofthe spread on the sensitivity value S, i.e., until the overall responseof the sensor device 1 shows the expected or nominal value.

FIG. 4 is a schematic illustration of an electronic device 30 in whichthe magnetic-field sensor 2 and the associated reading circuit 3 findapplication, for example to provide a magnetometer.

The magnetic-field sensor 2 and the corresponding reading circuit 3 canbe made, with semiconductor micromachining techniques, inside respectivedice of semiconductor material, for example silicon (the reading circuit3 being provided as ASIC—Application-Specific Integrated Circuit) andare integrated within a same package (so as to form the sensor device 1in a single chip).

The electronic device 30 comprises a control unit (a microcontroller ormicroprocessor, or similar computing and processing tool) 32, connectedto the reading circuit 3 of the sensor device 1, in particular forcontrolling the operations of the reading circuit (such as execution,when desired, of the calibration procedure), and for acquiring andpossibly processing further the (analog or digital) output signal Voutsupplied at the output from the reading circuit 3, once the gain G ofthe amplification stage 6 has been calibrated. The electronic device 30further comprises a memory 34 (optional) and a supply source 36,connected to the sensor device 1, to the magnetic-field sensor 2, to thecontrol unit 32, and to the memory 34, to provide the power supply fortheir operation; the supply source 36 may comprise, for example, abattery.

In a way not illustrated, the electronic device 30 can comprise furthermagnetic-field sensors 2 and corresponding reading circuits 3 in orderto carry out a detection along a number of measurement axes, for examplea set of three cartesian axes x, y, z, to obtain a triaxial system fordetection of external magnetic fields. In a known way, threemagnetic-field sensors 2 are sufficient to identify three spatialcomponents of an external magnetic field H_(e), thus uniquelyidentifying the direction and intensity thereof. In this case, theelectronic device 30 may further comprise a position-detection system,for example including an accelerometer, configured for detectingorientation of the electronic device 30 with respect to the Earth'ssurface.

Various advantages of embodiments of the circuit and of the readingmethod according to the present description emerge from the foregoingdescription.

In particular, the presence of the calibration stage 12 in the readingcircuit 3 associated to the magnetic-field sensor 2 enables an automaticcalibration of the spread of sensitivity of the sensor to be carriedout, without any need for interventions from the outside. Calibration isadvantageously carried out within the same sensor device 1, or in anycase within the electronic device 30 that incorporates the sensor device1, without there being required any complex external testing equipment.The calibration procedures can be implemented also by the end user, ifdesired by the applications.

Advantageously, the operations of calibration can be carried out at thelevel of the ASIC (i.e., in the integrated circuit associated, in thesame package, to the magnetic-field sensor 2), without there beingrequired further processing operations by an external electronic unit.

In addition, the use of the offset strap 4, integrated in themagnetic-field sensor 2, enables (by generation of a controlled current)generation of a field of a known value for the operations ofcalibration, without requiring a controlled external environment inwhich to perform the same operations (the algorithm described does notrequire in fact the knowledge of the value of the external magneticfield H_(e)).

In general, the circuit and the method described enable elimination (orat least reduction) of the deviation of the value of the output signalVout of the sensor device 1 with respect to the expected value, even inthe presence of a high spread (for example, equal to 20%) of thesensitivity of the magnetic-field sensor 2.

Finally, it is clear that modifications and variations may be made towhat has been described and illustrated herein, without therebydeparting from the scope of the present disclosure.

In particular, it is evident that the circuit implementation of thecalibration stage 12 may vary with respect to what has been describedand illustrated herein; a variable-gain resistive network may, forexample, be provided, associated to the amplifier 8; the amplifier 8 mayhave a single-differential output (i.e., a single output on which theoutput signal Vout is present). The calibration algorithm may differfrom the one described and illustrated; for example, the algorithm mayenvisage the use of a dichotomic technique (of a type in itself known)for identifying by successive approximations the appropriate value to beassigned to the gain G of the amplification stage 6 for compensating thevariation of sensitivity of the magnetic-field sensor 2.

Embodiments of the methods and circuits according to the presentdisclosure may moreover be used for compensating the offset of furthermagnetic-field sensors comprising magnetoresistive elements (or ingeneral at least one magnetoresistive element) in a configuration thatmay even be different from the one described and illustrated.

Some embodiments may take the form of or comprise computer programproducts. For example, according to one embodiment there is provided acomputer readable medium comprising a computer program adapted toperform one or more of the methods or functions described above. Themedium may be a non-transitory medium such as a physical storage medium,for example, a Read Only Memory (ROM) chip, or a disk such as a DigitalVersatile Disk (DVD-ROM), Compact Disk (CD-ROM), a hard disk, a memory,a network, or a portable media article to be read by an appropriatedrive or via an appropriate connection, including as encoded in one ormore barcodes or other related codes stored on one or more suchcomputer-readable mediums and being readable by an appropriate readerdevice.

Furthermore, in some embodiments, some or all of the systems and/ormodules may be implemented or provided in other manners, such as atleast partially in firmware and/or hardware, including, but not limitedto, one or more application-specific integrated circuits (ASICs),digital signal processors, discrete circuitry, logic gates, statemachines, standard integrated circuits, controllers (e.g., programmed byexecuting appropriate instructions, and including microcontrollersand/or embedded controllers), field-programmable gate arrays (FPGAs),complex programmable logic devices (CPLDs), etc., as well as devicesthat employ RFID technology, and various combinations thereof.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary to employ concepts of the various patents, application andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A reading circuit, comprising: an amplification stage configured toreceive a magnetic field detection signal of a magnetic field sensor andto generate an output signal as a function of the magnetic fielddetection signal and of an amplification gain of the amplificationstage; and a calibration stage configured to control the amplificationgain of the amplification stage based on an indication of a detectionsensitivity of the magnetic field sensor.
 2. The circuit according toclaim 1 wherein said calibration stage is configured to: measure a valueof the output signal associated with a controlled magnetic field havinga known value; determine an effective value of said detectionsensitivity as a function of said measured value; and control theamplification gain based on said effective value of said detectionsensitivity.
 3. The circuit according to claim 2 wherein saidcalibration stage is configured to generate an excitation current toexcite a magnetization element of the magnetic field sensor and theamplification stage is configured to electrically couple to a firstmagnetoresistive element of the magnetic field sensor.
 4. The circuitaccording to claim 1 wherein said calibration stage is configured to:measure at least a first value of said output signal, in the presence ofan external magnetic field and in the absence of a controlled magneticfield; measure at least a second value of said output signal, in thepresence both of said external magnetic field and of said controlledmagnetic field; determine an effective value of said detectionsensitivity as a function of said first and second measured values; andcontrol the amplification gain based on said effective value of saiddetection sensitivity.
 5. The circuit according to claim 4 wherein saidcalibration stage is configured to determine a difference between saidfirst measured value and said second measured value of said outputsignal.
 6. The circuit according to claim 1 wherein said amplificationstage comprises an amplifier, having at least one input configured toreceive said magnetic field detection signal and at least one outputconfigured to supply said output signal; and wherein said calibrationstage comprises a gain-variation unit, coupled to said amplifier andconfigured to vary a gain of the amplifier between the at least oneinput and the at least one output of the amplifier.
 7. The circuitaccording to claim 6 wherein said amplifier comprises a gain network,coupled to said at least one input and to said at least one output, andsaid gain-variation unit comprises an adjustable-impedance unit coupledto said gain network and having a selectable impedance value, whereinsaid calibration stage is configured to provide a gain-control signal tosaid gain-variation unit for selecting a value of said selectableimpedance.
 8. The circuit according to claim 1 wherein the magneticfield sensor is an anisotropic magnetoresistive (AMR) magnetic sensor,the amplification stage is configured to electrically couple to a bridgedetection structure formed from a plurality of magnetoresistive elementsof the magnetic field sensor, and said magnetic detection signal is anunbalancing signal of said bridge detection structure.
 9. A systemcomprising: a magnetic-field sensor configured to generate a magneticfield detection signal as a function of one or more magnetic fields; anda reading circuit having: an amplification stage coupled to themagnetic-field sensor and configured to generate an output signal as afunction of the magnetic field detection signal and of an amplificationgain of the amplification stage; and a calibration stage configured tocontrol the amplification gain of the amplification stage based on anindication of a detection sensitivity of the magnetic field sensor. 10.The system of claim 9, further comprising a control unit, coupled tosaid reading circuit and configured to receive said output signal. 11.The system according to claim 9 wherein said reading circuit is anapplication-specific integrated circuit housed in a same packagetogether with a die comprising said magnetic-field sensor.
 12. Thesystem of claim 9 wherein said calibration stage is configured to:measure a value of the output signal associated with a controlledmagnetic field having a known value; determine an effective value ofsaid detection sensitivity as a function of said measured value; andcontrol the amplification gain based on said effective value of saiddetection sensitivity.
 13. The system of claim 12 wherein saidcalibration stage is configured to generate an excitation current toexcite a magnetization element of the magnetic field sensor and theamplification stage is configured to electrically couple to a firstmagnetoresistive element of the magnetic field sensor.
 14. The system ofclaim 9 wherein said calibration stage is configured to: measure atleast a first value of said output signal, in the presence of anexternal magnetic field and in the absence of a controlled magneticfield; measure at least a second value of said output signal, in thepresence both of said external magnetic field and of said controlledmagnetic field; determine an effective value of said detectionsensitivity as a function of said first and second measured values; andcontrol the amplification gain based on said effective value of saiddetection sensitivity.
 15. The system of claim 14 wherein saidcalibration stage is configured to determine a difference between saidfirst measured value and said second measured value of said outputsignal.
 16. A method, comprising: receiving a signal of a magnetic-fieldsensor, the received signal being a function of a detected magneticfield and of a detection sensitivity of the magnetic-field sensor;controlling an amplification gain of a reading circuit to compensate fora variation of said detection sensitivity from a nominal detectionsensitivity value; and generating an output of the reading circuit as afunction of said received signal and of said amplification gain.
 17. Themethod of claim 16 wherein said controlling the amplification gaincomprises: measuring a value of the output signal associated with acontrolled magnetic field having a known value; determining an effectivevalue of said detection sensitivity as a function of said measuredvalue; and controlling the amplification gain based on said effectivevalue of said detection sensitivity.
 18. The method of claim 17 whereinsaid measuring a value of the output signal associated with a controlledmagnetic field comprises generating an excitation current to excite amagnetization element of the magnetic field sensor.
 19. The method ofclaim 16 wherein said controlling the amplification gain comprises:measuring at least a first value of said output signal, in the presenceof an external magnetic field and in the absence of a controlledmagnetic field; measuring at least a second value of said output signal,in the presence both of said external magnetic field and of saidcontrolled magnetic field; determining an effective value of saiddetection sensitivity as a function of said first and second measuredvalues; and controlling the amplification gain based on said effectivevalue of said detection sensitivity.
 20. The method of claim 19 whereinsaid determining said effective value comprises determining a differencebetween said first measured value and said second measured value of saidoutput signal.
 21. A system comprising: means for generating a magneticfield detection signal as a function of one or more magnetic fields;means for generating an output signal as a function of the magneticfield detection signal and an amplification gain; and means forcontrolling the amplification gain based on an indication of a detectionsensitivity of the means for generating the magnetic field detectionsignal.
 22. The system of claim 21 comprising an application-specificintegrated circuit including the means for generating the output signaland the means for controlling the amplication gain in a same packagetogether with a die comprising said means for generating the magneticfield detection signal.
 23. The system of claim 21 wherein said meansfor controlling the amplification gain is configured to: measure atleast a first value of said output signal, in the presence of anexternal magnetic field and in the absence of a controlled magneticfield; measure at least a second value of said output signal, in thepresence both of said external magnetic field and of said controlledmagnetic field; determine an effective value of said detectionsensitivity as a function of said first and second measured values; andcontrol the amplification gain based on said effective value of saiddetection sensitivity.
 24. The system of claim 23 wherein said means forcontrolling the amplification gain is configured to determine adifference between said first measured value and said second measuredvalue of said output signal.